Method of controlling interaction between pretreating and hydrocracking stages

ABSTRACT

A METHOD OF DETERMINING AND CONTROLLING THE CONCENTRATION OF POLYCYCLIC AROMATIC COMPOUNDS IN VARIOUS HYDROCARBON STREAMS OF A MULTI-STAGE PROCESS COMPRISING PRETREATING AND HYDROCRACKING IS DESCRIBED. THE METHOD COMPRISES EMPLOYING ULTRAVIOLET AND VISIBLE LIGHT TO DETERMINE THE CONCENTRATION OF THREE OR MORE CONDENSEDRING POLYCYCLIC AROMATICS IN HYDROCARBON STREAMS AND IN REPONSE TO THE MEASURED CONCENTRATION OF CONDENSED-RING POLYCYCLIC AROMATICS, CONTROL MEANS ARE OPERATED TO VARY THE RAW FEED COMPOSITION TO THE PROCESS COMPRISING FEED PRETREAT FOLLOWED BY HYDROCRACKING THEREOF, THE CONDITIONS EMPLOYED IN THE PRETREAT STAGE SO AS TO CONTROL THE CONCENTRATION OF POLYCYCLIC AROMATICS IN THE EFFUENT THEREFROM AND THE CONCENTRATION OF POLYCYCLIC AROMATICS IN THE HYDROCARBON CHARGE TO THE HYDROCRACKING STAGE OF THE PROCESS PARTICULARLY WHEN RECYCLE PRODUCE OF THE HYDROCRACKING STEP IS RETURNED THERETO.

United States Patent Int. Cl. Cg 23/00 US. Cl. 208-89 '5 Claims ABSTRACTOF THE DISCLOSURE A method of determining and controlling theconcentration of polycyclic aromatic compounds in various hydrocarbonstreams of a multi-stage process comprising pretreating andhydrocracking is described. The method comprises employing ultravioletand visible light to determine the concentration of three or morecondensedring polycyclic aromatics in hydrocarbon streams and inresponse to the measured concentration of condensed-ring polycyclicaromatics, control means are operated to vary the raw feed compositionto the process comprising feed pretreat followed by hydrocrackingthereof, the conditions employed in the pretreat stage so as to controlthe concentration of polycyclic aromatics in the efiluent therefrom andthe concentration of polycyclic aromatics in the hydrocarbon charge tothe hydrocracking stage of the process particularly when recycle produceof the hydrocracking step is returned thereto.

This application is a continuation of Ser. No. 710,901 filed Mar. 6,1968, now abandoned.

BACKGROUND OF THE INVENTION Recently, there has been increased incentiveto use hydrocracking processes to produce fuel products and especiallygasoline products. Hydrocracking is useful in converting relativelyrefractory feeds containing high boiling polycyclic aromatichydrocarbons or other high boiling hydrocarbons to lower boilingproducts such as gasoline without excessive gas or coke formation. Thesesame refractory feeds however when charged to a catalytic crackingprocess operated in the substantial absence of hydrogen gas, eifectexcessive coke formation on the catalyst, and thus seriously reduce thecatalyst activity and its ability to maintain product yield. Presenthydrocracking processes generally include at least two zones or stagesof catalytic treatment comprising a first hydrocarbon feedpretreating-hydrogenation stage followed by one or more stages ofhydrocracking.

The pretreating stage is employed to at least effect a reduction insulfur and nitrogen concentrations in the feed by converting compoundsof these elements to hydrogen sulfide and ammonia, respectively. It hasbeen found that nitrogen compounds usually found in hydrocracking feedsadversely reduce the cracking activity of most hydrocracking catalystsand often accelerate catalyst deactivation when present. When thisoccurs, frequent regenerations become necessary to maintain a desiredhydrocarbon conversion rate by hydrocracking. The deleterious effects ofnitrogen, sulfur and carbonaceous residue are particularly evident whenan amorphous or non-crystalline base hydrocracking catalyst is employedin the hydrocracking stages. Hydrocracking catalysts comprising acrystalline aluminosilicate are less adversely affected by nitrogen andsulfur concentrations in the feed than are the amorphous basehydrocracking catalysts; howevenmany sources of petroleum hydrocrackingfeeds contain high concentrations of or- Fee ganic nitrogen compoundsand in amounts which seriously adversely affect the activity of thesecatalysts. Thus, the hydrocarbon feed to be passed in contact with acrystalline aluminosilicate containing hydrocracking cata lyst alsooften requires a preliminary pretreating or hydrogenation step to reducethe sulfur and nitrogen concentrations of the feed to desired low levelsso as to achieve optimum hydrocracking catalyst performance.

A crystalline aluminosilicate containing hydrocracking catalystgenerally exhibits increased cracking activity for relatively largeportions of the feed, and an improved selectivity as compared to theamorphous-base catalyst. It is these characteristics of the crystallinealuminosilicate containing hydrocracking catalysts which have promotedtheir increased use in hydrocracking processes. However, the crystallinealuminosilicate containing hydrocracking catalysts suffer thedisadvantage of exhibiting a poor ability for cracking relatively highmolecular weight polycyclic hydrocarbons as compared to theamorphousbase hydrocracking catalyst. This is probably due tointraparticle diffusion limitations caused by the relatively small poresizes of the crystalline aluminosilicate. Thus, when employing acrystalline aluminosilicate containing hydrocracking catalyst, having a.pore size in the range of 6 to 15 angstroms, and when recycling at leasta portion of the heavier products of single-pass operation toextinction, it is necessary to control the amount of polycyclichydrocarbons in the feed and coming in contact with the crystallinealuminosilicate portion of the catalyst in the hydrocracking zone.Otherwise, the polycyclic hydrocarbon concentration will steadilyincrease in the recycle stream recovered from the hydrocracking zoneand, when recycled thereto, will adversely affect the crackingselectivity and activity of the crystalline aluminosilicate-basecatalysts. In the case of amorphous-base hydrocracking catalysts, it isprincipally their activity in the second stage that is adverselyaffected under hydrocracking conditions by excessive concentrations ofpolycyclic aromatic hydrocarbons.

The build-up of polycyclic aromatic hydrocarbons, for example, three ormore condensed ring compounds, in the hydrocracking stage causesincreased catalyst aging rates therein and thereby necessitates morefrequent catalyst regenerations. On the other hand, the catalystemployed in the raw feed pretreating-hydrogenation zone is much lessrapidly deactivated. by nitrogen and condensed ring polycyclic aromatichydrocarbon compounds when operated at selected pretreating conditionsthan either an amorphous-base hydrocracking catalyst, a crystallinealuminosilicate-base hydrocracking catalyst or a mixture thereof whenemployed at hydrocracking conditions. Thus, generally, pretreatingcatalysts need not be regenerated as frequently as hydrocrackingcatalysts, but as both stages of pretreating and hydrocracking are in asingle train, when the catalyst in one stage is being regenerated, theentire process sequence must be shut down. When it is necessary toregenerate the catalyst in only one stage to provide process downtimetherefor, substantial economic disadvantages for the process areintroduced. Greater process efiiciency and thus substantially improvedeconomics will result if the regeneration of both thehydrogenation-pretreating catalyst and the hydrocracking catalyst areaccomplished simultaneously since more eflicient use is made of theprocess capacity. It is thus desirable to improve-process efficiency bydecreasing the aging rate of the hydrocracking catalyst to a valuecommensurate with that of the pretreating-hydrogenation catalyst.

At the present time, hydrocracking catalyst aging rates are improved byselecting stocks having a limited concentration of polycyclic aromatic:hydrocarbons therein.

This feed selection, however, introduces considerable processdisadvantages by reducing the flexibility of the process for convertinga wide variety of hydrocarbon feeds. Furthermore, as the hydrocrackingcatalyst activity is reduced during on-stream time, its ability toconvert polycyclic aromatic hydrocarbons is significantly reduced andthe permissible amount of polycyclic aromatic hydrocarbons in the chargewill vary with on-stream time. While the hydrocracking catalyst may besufficiently active initially to convert the hydrocarbons in the feedother than those of the polycyclic aromatic type, this activity will berapidly impaired by the condensed ring component of the polycyclicaromatics in the feed and an undesirable build-up of insufficientlyconverted hydro carbons and polycyclic aromatic hydrocarbons in therecycle stream will result. Thus, it will become necessary to regeneratethe hydrocracking catalyst more often than desired to maintain itsactivity for hydrocarbon conversion including polycondensed aromatichydrocarbons.

SUMMARY OF THE INVENTION In accordance with the present invention, apetroleum hydrocarbon fresh feed material boiling within the range offrom about 400 F. to about 1100 F. and containing nitrogen, sulfur andpolycyclic compounds of three or more condensed ring components ispassed sequentially through a first feed hydrogenation-pretreating zoneand then through at least one second hydrocracking zone in the presenceof a suitable hydrocracking catalyst. The efiluent from thehydrogenation-pretreating zone is separated to recover hydrogenated andpartially cracked hydrocarbons from unreacted hydrogen, ammonia, waterand hydrogen sulfide. Separated hydrogen may be recycled to the processas desired. The hydrocarbon stream obtained from the pretreatedseparation step of acceptable polycyclic aromatic concentration is thendirected to hydrocracking treatment in one or more separatehydrocracking zones wherein it is further converted to lower boilingproducts such as gasoline. The hydrocracking effiuent is separated toseparately recover lower boiling desired products from a high-boilingrecycle stream comprising unreacted or partially converted hydrocarbonsincluding polycyclic aromatic constituents. The recycle stream of thehydrocracking step may be combined as desired with the hydrocarboneflluent obtained from the feed pretreating-hydrogenation zone to formthe acceptable hydrocarbon feed passed to the hydrocracking zone.

The efiiuent of the hydrogenation-pretreatment step, the hydrocrackingrecycle stream and the raw fresh feed to the process are monitoredsubstantially continuously or intermittently as desired so as toidentify the concentration of condensed ring polycyclic aromatichydrocarbons having more than three and preferably from about 3 to about7 unsaturated condensed rings therein. The measurements thus obtainedare then used to control conditions in the hydrocarbon feedpretreating-hydrogenation zone in a manner to limit the concentration ofpolycyclic aromatic hydrocarbons from building up in the hydrocrackingstep. The concentration of polycyclic aromatic condensed ring compoundsas measured by the maximum in the most prominent peak of the visible andultraviolet spectrum between about 3000 and about 5000 angstroms islimited in the hydrocarbon stream passed to the hydrocracking zone tothe following ranges. If that stream contains polycyclic condensed-ringaromatic compounds which significantly absorb light between about 4000and about 5000 A., the concentration of those compounds is limited tonot more than 350 p.p.m. and preferably the concentration is maintainedat less than about 100 p.p.m. If, on the other hand, the stream does notcontain polycyclic condensed-ring aromatics which absorb lightsignificantly between about 4000 and about 5000 A., then theconcentration of polycyclic condensed-ring aromatics which absorb lightbetween about 3000 and about 4000 A. is used for control and is limitedto not more than 4 10,000 ppm. and preferably the concentration ismaintained at less than about 1000 p.p.m.

A comment about asborption of light by aromatics seems appropriate atthis point in the discussion because it helps to explain some apparentwordiness required for precision in the discussion herein presented. Forexample, the polycyclic aromatic compound phenyl perylene contains 6aromatic rings. Five are condensed aromatic rings (i.e. rings having atleast 2 carbon atoms common to one another) and one is an uncondensedaromatic ring (i.e. a ring attached to but having no carbon atom oratoms in common with another ring). This compound absorbs light 1) as anaromatic having 5 condensed rings and (2) as a single ring aromatic.However, it does not absorb light near the Wave length at whichcorresponding 6 condensed-ring aromatic compounds absorb light. Inmatters related to absorption spectroscopy, therefore, a reference to apolycyclic aromatic generally in this specification does not identifythe particular number of condensed-ring constituents. A reference to anx-ring polycyclic aromatic, the other hand, means only that the aromatichas x condensed rings; it does not means that the aromatic only has xrings; it may or may not have more than x rings. Thus in an effort toclearly identify applicants invention and in view of the effect ofcondensed-ring structures on light absorption spectroscopy, the presentapplication is directed primarliy to referring to polycyclic aromaticcompounds in terms of their condensed-ring structure.

It is to be understood that the pretreating-hydrogenation zone and thehydrocracking zones of the present invention may be maintained withinone or more reactors.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 presents diagrammatically onearrangement of process steps for practicing the method of this inventioncomprising a hydrogenation-pretreat step followed by a hydrocrackingstep wherein different streams can be ana- Lyzed for condensed ringpolycyclic aromatic hydrocarons.

FIG. 2 presents a plot of data arranged to show the effect of reactortemperature on aromatics and nitrogen content of the pretreating stageefiluent.

FIG. 3 presents a plot of data arranged to show a relationship betweenpretreating stage performance with catalyst aging rate in the subsequenthydrocracking stage.

DESCRIPTION OF SPECIFIC EMBODIMENTS It has been found that bycontrolling the feed pretreating-hydrogenation conditions in response tothe concentration of polycyclic aromatics of three or morecondensed-ring structures tolerable in the hydrocarbon feed streampassed from the pretreating-hydrogenation step to the hydrocracking stepthat the hydrocracking catalyst can be more efficiently used foreffecting the conversion of the hydrocarbon charge to lower boilingdesired products. Furthermore, by controlling the severity of thepretreating conditions in accordance with this invention there isprovided a convenient method for improving the hydrocracking catalystaging rate without adversely affecting the process throughput rate.Thus, the present invention provides a convenient method and processwhich closely controls the catalyst aging rates in each of apretreating-hydrogenation step and one or more subsequent hydrocrackingsteps. In one embodiment of the invention, this control is employed sothat the catalysts of all stages will be in a condition to beregenerated simultaneously and thus eliminate the need for additionalproces down-time to separately regenerate only one of the catalysts.Therefore it can be seen that the method and process of this inventionprovides substantially economic advantages over presently knownhydrocracking processes.

The hydrogenation-pmtreating catalyst employed in the process of thisinvention generally comprises a hydrogenatron component dispersed on anamorphous-base and is generally characterized as a catalyst ofrelatively mild cracking activity and greater hydrogenating activity ascompared to an amorphous or crystalline aluminosilicate containinghydrocracking catalyst. Depending upon the activity of thepreheating-hydrogenation catalyst, the re action in thepreheating-hydrogenation zone can be either rate controlled orcontrolled by the hydrogenation-dehydrogenation equilibrium. For a givenpressure and contact time in the pretreating-hydrogenation zone, theconcentration of polycyclic aromatic hydrocarbon in the effluenttherefrom depends upon (1) temperature and (2) whether the reaction inthe pretreating-hydrogenation zone is controlled by the rate of reactionor by the hydrogenation-dehydrogenation equilibrium. In such a system,when the reaction is rate-controlled, the concentration of polycyclicaromatic hydrocarbons can be reduced by either increasing thetemperature or decreasing the space velocity. When the reaction iscontrolled by the hydrogenation-dehydrogenation equilibrium, on theother hand, the concentration of polycyclic aromatic hydrocarbons in thehydrogenation zone effluent is reduced by decreasing temperature.Whether the reaction is rate-controlled or equilibrium-controlled, theconcentration of polyclclic aromatic hydrocarbons in the hydrogenationstage efiiuent can be reduced by increasing the hydrogen partialpressure and/ or reducing the oil partial pressure in thehydrogenationpretreating zone. When the reaction is rate-controlled,reducing the temperature will cause the rate of conversion of thepolycyclic aromatics to be reduced. This is undesirable since theconcentration of polycyclic aromatic hydrocarbons is increased thereby.However, when the reaction is at equilibrium, a temperature increasewill be un desirable since the polycyclic aromatic concentration willincrease. Therefore, at a given hydrogen partial pressure and contacttime operating at the lowest possible temperature at which the reactionis equilibrium-controlled will reduce to a minimum the concentration ofpolycyclic aromatics in the pretreating-hydrogenation efiiuent.

As stated above, the concentration of polycyclic aromatic hydrocarbonscomprising three or more condensedring constituents and found in thepretreating-hydrogenation stage hydrocarbon efliuent can be reduced byincreasing hydrogen partial pressure during hydrogenation thereof.However, the pretreating-hydrogenation reactors are built to withstand acertain maximum pressure which is usually dictated by the economics ofthe process and the advantages to be gained by the increased pressureversus the cost of building the reactor of increased strength.Therefore, within the limits of a particular process design, thehydrogen partial pressure can be conveniently increased in thepretreating step by increasing the hydrogen circulation rate therein. Inaddition, the concentration of polycyclic aromatic hydrocarbons in theeffluent can be significantly reduced by regulating the temperature andspace velocity in the manner described herein. It is also within thescope of this invention to regulate the concentration of polycyclicaromatics found in the raw fresh feed to the pretreating step of theprocess in combination with the above-described process, thus utilizinga concept of this invention known and identified as feed forward controlof the process.

In the process of the present invention, any one of a number of knownmethods can be employed for ascertaining the concentration and number ofpolycyclic aromatics in a given hydrocarbon stream. Absorptionspectroscopy is presently known as one convenient method for identifyingthe concentration of condensed-ring polycyclic aromatic hydrocarbons ina hydrocarbon stream. The process of this invention will be specificallydescribed hereinafter in detail with reference to such a method ofdetection.

In accordance with this invention it has been found that the polycyclicaromatic hydrocarbons of which the concentration should be controlledare those having three or more, and preferably less than sevencondensed-ring constituents in the hydrocarbon structure. These-condensed rings can be substituted with either aliphatic or aromaticmoieties or can be substituted with heterocyclic rings. Polycyclicaromatic hydrocarbons of three or more condensed rings have very intenseadsorption bands in the visible and ultraviolet regions of theelectromagnetic spectrum in the range of from about 2000 to about 7000A. The portion of the electromagnetic spectrum in the ultraviolet andvisible regions which is of particular interest in the present contextis in the range of from about 3000 to about 5500 A. Those polycyclicaromatic hydrocarbon condensed-ring structures which are the mostcritical in the process of this invention have very intense adsorptionbands in the ultraviolet and visible electromagnetic wavelength range ofabout 3220 A., 3370 A., 3780 A., 4340 A., 4480 A., 4820 A. and 5225 A.The absorption bands of the pertinent polycyclic aromatic species willbe influenced by petroleum feed materials which contain mixtures ofcompounds of close chemical relationship and by chromophoric chemicalgroupings in the feed mixtures. However, the characteristic absorptionbands are sufficiently intense to be easily identified by those skilledin the art. The theory of absorption spectroscopy is well defined atpresent. It is reviewed in such treatises as Ultraviolet and VisibleSpectroscopy by Orchin and Friedel, and will be referred to only brieflyherein.

When an organic molecule is exposed to a continuous spectrum ofelectromagnetic radiation, radiation having certain wavelengths isabsorbed, causing excitation of the molecule to a higher energy state.The energy difference between the lower and higher energy statesdetermines the wavelength of the absorbed light. Many factors influencethe differences between energy states. The spectra to be dealt with inthe present invention are adsorption spectra which result largely fromtransitions of the electrons of molecules to various energy states.According to quantum mechanics, these energy states have discrete valuescorresponding to integral multiples of a unit of energy. The values ofthese discrete quantum mechanical energy levels depend on the molecularstructure of the absorbing species. For example, certain polycyclicaromatic hydrocarbons in petroleum absorb strongly at very specificwavelengths in the visible and ultraviolet region and exhibit a uniqueabsorption pattern.

According to Beers Law, the amount of light absorbed in a layer isproportional to the number (concentration) of absorbing molecules in alayer of absorbing material. According to Lamberts Law, the amount oflight absorbed is proportional to the thickness of the layer. Thecombination of these two laws is expressed mathematically as follows:

I=Transmitted light intensity I =Incident light intensity E=M0larextinction coefiicient b=Cell thickness, i.e. layer thicknessc=Concentration of the absorbing species I M=M0lecular weight Thequantity the apparent K value is proportional to the cencentration.

of those aromatics. On the other hand, the K value of a mixture ofmaterials absorbing at 4340 A., for example in an otherwise transparentdiluent, depends on-some combination of the K values of the individualcomponents of that mixture. When that mixture is approximately constantin composition, the amount of absorption is proportional to theconcentrations of mixed aromatics.

A system of instrumentation which may typically be used in thisinvention, is the Du Pont 400 photomertic analyzer. This instrument isfully described in a series of publications by the Instrument ProductsDivision of the Du Pont Company. The sample stream flows continuouslythrough the sample cell. The term split-beam refers to the division ofthe incident diffuse light into reference and measuring beams afterlight has passed through the sample. In a typical arrangement, radiationfrom a selected light source passes through the sample and then to aphotometer unit Where it is split by a semi-transparent mirror into twobeams. One beam is directed to the measuring photometer through anoptical filter which removes all wavelengths except the measuredwavelength. This wavelength is strongly absorbed by this sample. Thesecond beam falls on the reference photoelectric tube after passing anoptical filter which transmits only the reference wavelength. The latteris absorbed only weakly or not at all by the constituent in the samplecell. The photoelectric tubes translate these intensities toproportional elecrical currents in the amplifier. In the amplifier,correction is made (by logarithmic amplifiers) for the logarithmicrelationship between the ratio of the intensities and concentration (orthickness) in accordance with Beers and Lamberts Laws. The output of thelogarithmic amplifier is, therefore, linear with sample concentration.Use of the split-beam technique implies that changes in light sourceintensity during analysis, or the presence of bubbles or particulatematter sample cell have virtually no effect on the accuracy of theanalysis. The presence of any extraneous matter will bias both thereference and measuring wavelengths equally. Online analyses can beconducted at pressures up to 750 p.s.i. and temperatures up to 700 F.Sample cell thickness range from .002 inch to 24 feet. Response times aslow as about .001 second can be attained with suitable cell design.Additional information regarding ultraviolet instrumentation can befound in Ultraviolet Spectra of Aromatic Compounds by Friedel andOrchin. It is within the scope of this' invention to employ eithercontinuous or intermittent process control.

While the present invention will be discussed in detail with referenceto methods of analysis comprising absorption spectroscopy, it is to beunderstood that other available methods for determining theconcentration of polycyclic or poly-condsensed aromatic hydrocarbonsgenerally, or those of 3 to 7 condensed-ring structures, can

' be employed with equal facility. Suitable analytical methods which canbe employed either alone or in combination are gradient elutionchromatography, mass spectrometry, infrared absorption spectroscopy,gas-liquid chromatography and magnetic resonance including nuclearresonance, electron paramagnetic resonance and quadruple resonance.

The operating conditions employed in the process of the presentinvention depend upon the particular catalysts employed in the varioussteps of the process. Each catalyst composition has, for example, aparticular aging rate which is dependent upon its interaction with thetype of feed employed and the reaction conditions employed. For example,zeolite-base hydrocracking catalysts have relatively high aging rates inthe presence of relatively high concentrations of polycyclic aromatichydrocarbons, having several condensed rings and lower aging rates inthe presence of organo-nitrogen compounds, as compared with anamorphous-base hydrocracking catalyst. Therefore, the operatingconditions in the pretreat-hydrogenation step will vary depending uponthe catalyst employed,

the hydrogenation conditions most suitable for'the catalyst and the feedemployed. It is desirable however to vary the conditions in thehydrogenation feed pretreat step so that the catalysts in each of thehydrogenation step and the subsequent hydrocracking step will requireregeneration at or about the same time. This can be effected bydetermining the catalyst aging rate with a particular hydrocarbon feedand then suitably regulating the pretreating-hydrogenation stepconditions. In place of or in addition to the pretreater feedmonitoring, the component feed streams employed to make up thepretreater feed can also be monitored for feed forward control of thepretreater; and the blending of these streams can be controlled by saidmonitoring. The process of the present invention can be controlledmanually, or automatically such as by a computer control which isresponsive to the visible and ultraviolet measurements for undesiredpolycyclic material above identified and obtained, whether measuredcontinuously or intermittently.

The process of the present invention can also be monitored andcontrolled by measuring the concentration of condensed-ring polycyclicaromatic hydrocarbons in (1) the feed to the pretreating-hydrogenatingstep, (2) the recycle stream of the subsequent hydrocracking stage orstages, (3) the hydrocracking-stage feed stream made up of both therecycle stream and the hydrogenation step effluent stream, or any one ofthese streams alone or in combination with another as discussed above.More precise process control can be obtained by measuring theconcentration of condensed-ring polycyclic aromatics in all threestreams. It is within the scope of this invention to measure theconcentration of polycyclic aromatics in the raw feed charged to thepretreating-hydrogenation zone and to adjust the feed composition inthis respect in order to obtain approximately equal catalyst on-streamtime and deactivation in all the reaction stages.

The amorphous-base hydrogenation-pretreating catalyst employed comprisesone or more hydrogenation com ponents dispersed in and on anamorphous-base having cracking activity and pores of a size above about20 A.,

and preferably between about 30 and about 500 A. The

hydrogenation components which can be employed therewith include theGroup VI-B and Group VIII metals of the Periodic Table as well as theiroxides, their sulfides, or mixtures thereof. The Group VI-B metals whichcan be employed include chromium, molybdenum and tungsten while theGroup VIII metals which can be employed include iron, nickel, cobalt,the platinum type metals such as platinum, iridium, osmium; thepalladium type metals such as palladium, rhodium and ruthenium andmixtures thereof. Among the preferred hydrogenation components areincluded nickel-tungsten sulfide, nickel sulfide, cobaltmolybdenumsulfide, nickel-cobalt-molybdenum sulfide, platinum and palladiumsulfides, and platinum and palladium. The amorphous-base componentswhich can be used therewith include the oxides of metals of Groups II-A,IIIA, and IV-B of the Periodic Table as well as silica or mixturesthereof. Examples of amorphous bases which can be employed includesilica-alumina, silicazirconia, silica-zirconia-alumina,silica-magnesia, silica, alumina and the like. The amorphous-basecomponents employed are those having a cracking activity index betweenabout 15 and about 45 and preferably between about 20 and about 35 asmeasured by the Cat A test described by Alexander and Shirnp in NationalPetroleum News, 36 page R-537 (Aug. 2, 1944), and these specificationsobtain before the catalyst has been contacted with ammonia ornitrogen-containing organic compounds at hydrocracking conditions.

The hydrogenation metal component of the pretreating catalyst isemployed in amounts and under conditions selected to effect substantialhydrogenation reaction; that is, hydrogenation in amounts sufficient toconvert a substantial majority of any organic nitrogen and sulfurcompounds in the feed to ammonia and hydrogen sulfide respectively. Itis important to this invention that the hydrogenation activity besufficient to convert a substantial proportion of the polycycliccondensed aromatic compounds to the corresponding saturated compounds.The amount of hydrogenation component employed in the pretreatingcatalyst depends upon the hydrogenation activity of the particularcomponent employed and comprises from about 0.1 to about 45 weightpercent, based upon the weight of the amorphous cracking base. Whennickel or metals such as platinum or palladium are employed, preferablyfrom about 0.1 to about 6 weight percent and more preferably from about0.2 to about 2 weight percent are used based upon the weight of theamorphous base. When the hydrogenation component is other than nickel,platinum or palladium, it is preferred to employ from about 8 to about50 weight percent thereof calculated as metal oxides and based upon theweight of the amorphous base. The hydrogenation component can beintroduced into the amorphous base by impregnation, by coprecipitationon the base surface, by admixture, by ion exchange or by other methodswell-known in the art.

In the hydrogenation-pretreating step, the temperature is maintained inthe range of between about 600 F. and about 900 F., preferably betweenabout 650 F. and about 800 F., a hydrogen partial pressure in the rangebetween about 1000 p.s.i.g. and 3500 p.s.i.g., preferably about 1500p.s.i.g. and about 2500 p.s.i.g., with a liquid hourly space velocitybetween about 0.2 v./hr./v. and about 5/v./hr./v., and a hydrogencirculation rate of between about 1000 s.c.f./b. and about 20,000s.c.f./b., preferably between about 5000 s.c.f./b. and about 10,000s.c.f./b.

The hydrocracking catalysts employed herein may comprise one or morehydrogenation components in combination with a support material havingcracking activity such as a silicious cracking base, a crystallinealuminosilicate material or mixtures thereof having cracking activity.It is to be understood that while the crystalline aluminosilicatecracking component can be employed alone or as the sole support for thehydrogenation component, it can also be employed in association with anamorphous silicious cracking base such as described below. The ratiobetween the amount of crystalline aluminosilicate material and amorphouscracking base may vary considerably depending upon the activity of eachand may be in a range of from about 0 to 100% and preferably from about3 to about 80%.

The crystalline aluminosilicate cracking component of the hydrocrackingcatalyst is structurally characterized by having uniformly dimensionedpores formed b alumina and silica tetrahedra. For purposes of thepresent invention it is desirable to employ crystalline aluminosilicateshaving pore size openings between about 6 A. and A.

Both synthetic and naturally occurring crystalline aluminosilicatematerials may be used. Among those which may be used are the syntheticcrystalline aluminosilicates such as zeolites X, Y, B, L and T, andnaturall occurring materials such as faujasite, mordenite, chabazite,erionite, offretite, and others. These aluminosilicates are preferablyused in a form characterized by a low sodium or alkali metal content,below about five weight percent, and preferably below about two weightpercent, calculated as alkali metal oxide, based upon the weight of thealuminosilicate.

Such materials are prepared by base exchange with fluid containingmetal-bearing ions which are exchangeable with sodium or other alkalimetal ions in the manner described in Plank et al., US. patents,3,140,249, 3,140,253 and others to obtain a selective cracking catalystof high activity. Among the metallic cations which can be so introducedto enhance the cracking activity of the aluminosilicate are those of theGroups I-B through VIII of the Periodic Table, as well as the rareearths. Also, the alkali metal can be removed from the aluminosilicateby base exchanging with a hydrogen-containing cation such as theammonium or-the tetraalkylammonium ion.

followed by treatment to obtain the catayst in the hy- 10 drogen form.Further the aluminosilicate can be base exchanged in a manner to replacethe alkali metal cation with a mixture of the above metal cations or amixture of the above metal cations with hydrogen cation. The preferredforms of the crystalline aluminosilicate are those containing rare earthmetal cations, rare earth metal cations and hydrogen cations, palladiumions or nickel cations and hydrogen ions, palladium ions or nickelcations and rare earth cations, palladium ions or nickel cations andrare earth ions and hydrogen ions, since these forms of the materialexhibit high cracking activity and good selectivity. The remainingalkali metal content, calculated as metal oxide should be below about 5weight percent and preferably below about 2 weight percent to obtain thedesired cracking activity and selectivity.

The amorphous-base component of the hydrocracking catalyst ischaracterized by having a higher hydrocracking activity than that of thepretreating catalyst, a pore size above about 20 A., and preferably of asize in a range selected from about 30 and about 500 A. Theamorphousbase cracking components which can be used herein include theoxides of metals of Groups ILA, III-A, and IV-B of the Periodic Table aswell as silica or mixtures thereof. Examples of amorphous siliciouscracking bases which can be employed herein include silica-alumina,silica-zirconia, silica-zirconia-alumina, silica-magnesia, silica,alumina and the like. The amorphous-base components employed herein arethose having a cracking activity index between about 20 and about 60 andpreferably at least about 30 as measured by the Cat A test described byAlexander and Shimp in National Petroleum News, 36 page R-537 (Aug. 2,1944).

The hydrogenation metal component of the hydrocracking catalyst may beemployed in amounts in the range of from about 0.1 to about 45 weightpercent thereof based upon the weight or amount of the crackingcomponent, the type of cracking component and the hydrogenation metalemployed. The hydrogenation component may be introduced into thecracking component by ion exchange, by impregnation, as a physicaladmixture, or by other methods known to the art.

Nickel or metals such as platinum or palladium are employed in amountspreferably from about 0.1 to about 6 weight percent, and more preferablyfrom about 0.2 to about 3 Weight percent, based upon the Weight of thezeolitic base. When the hydrogenation metal component is other thannickel, platinum or palladium, it is preferred to employ from about 6 toabout 30 weight percent thereof, calculated as metal oxides and basedupon the crystalline aluminosilicate cracking base weight and from about8 to about 50 weight percent calculated as metal oxides and based on theamorphous cracking catalyst base weight. Hydrogenation components suchas nickel sulfide and tungsten sulfide mixtures in amounts of betweenabout 5 and about 15 weight percent of the nickel and tungsten metals,platinum in amounts of from about 0.1 and about 5 weight percentcombined with a zeolite X or a zeolite Y containing rare earth,hydrogen, or a mixture of rare earth and hydrogen cations in combinationwith a silicious cracking base are very effective hydrocrackingcatalysts.

The hydrocracking conversion conditions are selected so as to effectconversion of the hydrocarbon feed in the range of from about 20 toabout volume percent perpass to products boiling below 400 F. Generally,conditions are maintained at a temperature in the range of from about450 F. to about 900 F, preferably from about 550 F. to about 750 F; ahydrogen partial pressure in the range of from about 500 p.s.i.g. toabout 3,000 p.s.i.g., preferably from about 1,000 p.s.i.g. to about:2,500 p.s.i.g.; a space velocity in the range of from about 0.1 to about10 v./hr./v. and preferably from about 1 to about 5 v./hr./v., and at ahydrogen circulation rate in the range of from about 1,000 to about20,000 s.c.f./b. and preferably from about 3,000 to about 8,000s.c.f./b.

The hydrocarbon feeds which can be processed by this invention toadvantage are those distillates boiling in the range of between about400 F. and about 1100 F. or residual fractions which are essentiallyfree of ash and asphaltic constituents. Hydrocarbon feeds which can beemployed include virgin heavy vacuum gas oils, coker gas oils, gas oilfrom catalytic cracking processes, the heavy aromatic extracts obtainedby furfural extracting high boiling hydrocarbons such as light, medium,and heavy virgin gas oils, cracked gas oils, or mixtures thereof.

FIG. 1 presents diagrammatically one arrangement of process steps forpracticing the method of this invention comprising ahydrogenation-pretreat step followed by a hydrocracking step.

FIG. 2 presents a plot of data arranged to show the effect of reactortemperature on aromatics and nitrogen content of the pretreating stageeflluent.

FIG. 3 presents a plot of data arranged to show a relationship betweenpretreating stage performance with catalyst aging rate in the subsequenthydrocracking stage.

Referring to FIG. 1 by way of example a fresh gas oil feed containingpolycyclic aromatics is introduced to the process by conduit 2. Thehydrocarbon fed may be mixed with a hydrocarbon stream having arelatively higher concentration of polycyclic aromatics introduced byconduit 4. The resultant hydrocarbon feed is directed through conduit 6to a heater 8 wherein it is preheated to an ele vated temperaturesuitable for use in the pretreating-hydrogenation step. The preheatedfeed is recovered from heater 8 and thereafter passed topretreating-hydrogenation reactor 10 by conduit 12. Inprctreating-hydrogenation reactor 10, the fresh feed is reacted withhydrogen introduced to the process by conduit 14. The organo-nitrogenand sulfur compounds in the hydrocarbon feed passed to reactor 10 areconverted to ammonia and hydrogen sulfide respectively. Polycyclicaromatics compounds in the hydrocarbon feed are saturated and somecracking of the charge is also effected. The efiiuent of thepretreatinghydrogenation reactor 10 is removed through conduit 16 anddirected to a separator 18. Hydrogen rich gas is removed from separator18 through conduit '20 for recycle to the process. Fresh hydrogen richgas in conduit 22 may be combined with the recycle gas in conduit andthe resultant mixture thus formed directed to thehydrogenation-pretreating reactor through conduit 14. The remainingefliuent is removed from hydrogen separator 18 and directed throughconduit 24 to a stripping unit 26 wherein ammonia, hydrogen sulfide andwater are separated for removal through conduit 28. The hydrogenatedhydrocarbon stream from which nitrogen and sulfur compounds have beenremoved is withdrawn from stripping unit 26 through conduit 30, mixedwith recycle hydrocarbons from conduit 32 and directed through conduits34 and 48 to hydrocracking reactor 36. In hydrocracking reactor 36, thehydrocarbons except for aromatic moieties are converted underhydrocracking conditions to lower boiling hydrocarbons. The effluentfrom hydrocracking reactor 36 is removed through conduit 38 and directedto a separation zone 40. In separation zone 40, the product efliuent ofhydrocracking is separated to recover a hydrogen rich recycle stream,unconverted hydrocarbons, and gasoline product material. Gasolineproduct material is recovered through conduit 42. It is to be understoodthat several different product materials may be recovered in addition togasoline, such as jet fuels and fuel oils. The unconverted hydrocarbonsboiling above the product boiling range materials obtained fromseparation zone 40, is removed through conduit 32 and combined with thehydrogenated feed in conduit for passage to reactor 36 by conduits 34.Hydrogen rich gas in conduit 44 is mixed with fresh hydrogen rich gasintroduced through conduit 46 and the resultant mixture is directedthrough conduit 48 to hydrocracking reactor 36.

In the arrangement of FIG. 1 provisions are made for determining theconcentration of polycyclic aromatics in any one of four streams.Samples of the hydrocarbon feed in conduit 34 are taken through line 50.Samples of the hydrocarbon feed in conduit 32 are taken through line 52.Samples of the hydrocarbon feed in conduit 30 are taken through line 54.Samples of the hydrocarbon feed in conduit 6 are taken through line 56.Each of the hydrocarbon streams may be analyzed for polycyclic aro maticconcentration at the analysis step 58, and the information obtained isthen directed to process control 60. Signals from process control 60 canbe directed to vary the temperature, space velocity, hydrogen partialpressure or type of feed passed to the pretreating-hydrogenationreactor. The hydrogen partial pressure may be varied by opening orclosing valve 62 in response to a signal through line 64. The spacevelocity in hydrogenation reactor 10 may be varied by opening or closingvalve 66 in response to a signal passed through line 68 and 70. Thetemperature of the feed preheat and thus the hydrogenation reactor 10may be varied by controlling valve '72 which supplies combustion fuelthrough conduit 74 to the preheater 8. The valve 72 is controlled inresponse to a signal from control 60 through lines 68, 76 and 78. Thepolycyclic aromatic concentration in the fresh feed in conduit 12 isvaried by opening or closing valve 80 in response to a signal obtainedfrom control 60 through lines 68 and 76. The signals from control 60 canbe directed to the desired valves in any manner as for exampleelectronically, pneu matically, or mechanically.

Referring now to FIG. 2 by way of example, there is shown graphicallythe relation that has been found to exist between the temperatureemployed in the hydrogenation-pretreating stage and the condensed-ringpolycyclic aromatics content of the stabilized liquid effiuent obtainedtherefrom as indexed by absorption of light at a wavelength of about4480 A. by that eflluent. There is also graphically shown in FIG. 2 therelation that was found to exist between temperature in thehydrogenation-pretreating stage and the organically combined nitrogencontent of the hydrogenation-pretreater eflluent. The graphicalrepresentation of FIG. 2 is based on data obtained from processing acharge stock boiling in the range of from about 550 F. to about 950 F.and comprising a mixture of coker gas oils, catalytic cracking cyclestocks and an aromatic fraction obtained from furfural extraction of aheavy catalytic cracking cycle stock. This charge stock contained about1000 p.p.m. by weight of organically combined nitrogen, and it wasprocessed over a commercial pretreating catalyst at 2000 p.s.i.g. totalpressure, 1.2 liquid hourly space velocity, and 7000 s.c.f. of hydrogencirculation per barrel of liquid feed.

FIG. 2 shows that there is not a one-to-one correspondence between thecondensed-ring polycyclic aromatics content and the organically combinednitrogen content of the hydrogenation-pretreater efliuent. Rather it isshown that at a given condensed-ring polycyclic aromatics content, therecan be two different nitrogen contents depending on whether thepretreater eflluent was made at a temperature to the right or to theleft of that corresponding to the minimum in the graph of polycyclicscontent as a function of reactor temperature in FIG. 2. Point A on thecurve of FIG. 2 represents the lowest temperature at whichhydrogenation-dehydrogenation equilibrium is obtained for the catalystemployed. When the hydrogenadon-pretreating reactor is operated in amanner wherein the rate of reaction is controlling, the temperature-comcentration relationship is represented by the curve to the left of PointA. When operating under these conditions, the polycyclic aromaticcondensed-ring concentration can be decreased by either increasingtemperature or decreasing space velocity. On the other hand, when thereaction in the pretreating-hydrogenation reactor is controlled by thehydrogenation-dehydrogenation equilibrium, the temperature-concentrationrelationship is represented by the curve to the right of Point A. Whenoperating under these conditions, the polycyclic aromatic condensed-ringconcentration can be reduced by decreasing temperature.

Referring now to FIG. 3, there is shown graphically a relationship thatexists between the intensity with which the stabilized effluent liquidfrom the first-stage hydrogenation-pretreater absorbs light at awavelength of 4480 A. and the catalyst aging rate in a subsequenthydrocracking step. The data obtained with two different catalysts arepresented; one catalyst being formed of an amorphousbase having crackingactivity as support for the hydrogenation component, and the othercomprising a mixture of an amorphous-base having cracking activity and acrystalline-aluminosilicate base having cracking activity being used assupport for the hydrogenation component. The intensity of absorption ofthe light at 4480 A. by the pretreater effluent obtained from each ofthe catalysts is plotted, for convenience, as 1000 times the lightabsorption factor at that wavelength. The charge stocks used to producethe data reflected in FIG. 3 were all liquid efiluents containing about1 p.p.m. by weight of organically combined nitrogen obtained by thepretreating-stage operation under the conditions and with thepretreating catalyst of FIG. 2. The temperature at which the chargestocks for FIG. 3 were obtained varied with pretreating catalystage(e.g. time the pretreater was on stream and/ or degree of catalystdeactivation). Depending on that temperature, the charge stocks of FIG.3 had different polycyclic aromatics contents even though they all had 1p.p.m. by weight of organically combined nitrogen.

All the results represented in FIG. 3 are for 1 p.'p.m. N in thepretreater efliuent; it is therefore apparent that the specification ofthe nitrogen content of the liquid efliuent of the pretreating stage isnot sufiicient of itself to guarantee control over the catalyst agingrate in the subsequent hydrocracking stages. Particularly, it has beenshown that, at a fixed nitrogen content or level in the pretreaterefiiuent, catalyst aging in a subsequent hydrocracking stage depends onthe polycyclic aromatics content, as indexed by light absorption, ofthat effluent; and that dependence on polycyclic aromatics content hasbeen found to vary from catalyst to catalyst.

To illustrate our invention more clearly, it will be assumed that acatalyst aging rate of not more than 0.3 F. per day is desired in thehydrocracking stages of the process. From FIG. 3 we find that we mustoperate the preceding pretreating stage to produce an efliuent producthaving a light absorption factor of not more than 3X10- if we use theamorphous-base catalyst shown in FIG. 3 inthe hydrocracking stages ofthe process, and of not more than 8 10- if we use the mixedcrystallineamorphous base catalyst of FIG. 3 in the hydrocracking stagesof the process. Suppose that we use the amorphousbase catalyst in thehydrocracking stages of the process. Then we find from FIG. 2 that.satisfactory pretreatingstage effluent can be made at temperaturesbetween 675 F. and 715 F. under the conditions of FIG. 2 with thecatalyst of FIG. 2 at the degree of aging which that catalyst hadexperienced when the data of FIG. 2 were obtained. However, as thecatalyst of FIG. 2 ages further, the line representing nitrogen contentin FIG. 2 will shift to the right; the descending leg of the U-shapedcurve of light absorption factor will shift to the right whereas theascending leg of that curve will remain fixed; and the minimum value ofthe absorption factor will slowly increase. Thus, as the pretreatingcatalyst ages, the lower limit of the temperature ranges correspondingto satisfactory pretreating stage effiuent will increase while the upperlimit will remain constant, and the temperature range for satisfactorypretreating will become correspondingly narrower.

From the foregoing, it will be evident that the quantitative numericalresults presented in FIG. 2 change as the pretreating-stage catalystages, and therefore cannot be used per se to insure production ofsatisfactory pretreater efiiuent throughout the total life of thepretreating catalyst. The qualitative aspects of the results in FIG. 2,

however, do indicate the method of this invention for producing apretreating-stage effluent of a quality and composition, at least withrespect to the polycyclic aromatic condensedqing concentrations, whichwill be satisfactory as hydrocracking-stage feed. As we noted before,FIG. 3 shows that we must operate the pretreating stage to produce anelfluent having a light absorption factor of not more than 3 10I- if weuse the amorphous-base catalyst of FIG. 3 at an aging rate of 03 F. perday in the hydrocracking stages under the conditions of FIG. 3. Assumingfor purpose of illustration that our visible and ultraviolet lightmonitor on the pretreating-stage effluent shows that the effluent has anabsorption factor of 4X 10" we are not able to use this informationquantitatively with respect to FIG. 2 because our pretreating catalystis not necessarily at the same age as it was when the data of FIG. 2were obtained. However, we do know from FIG. 2 that raisingpretreating-stage temperature will bring a desired reduction in thelight absorption factor if the result given by the light absorptionmonitor lies on the descending leg of the U-shaped curve of FIG. 2.Therefore, either manually or automatically, and as a generated functionof the light absorption factor, we can raise the pretreating-stagetemperature and watch for a suitable effect on the light absorptionfactor of the pretreater efiluent. If the absorption factor comes down,we continue raising temperature until the absorption factor just fallsto 3 l0- at which point the pretreater effluent will be satisfactory asa hydrocracking-stage feed insofar as hydrocracking catalyst aging isconcerned. If, on the other hand, the absorption factor goes up astemperature is raised, we know that we are on the ascending leg of theU-shaped curve of FIG. 2. Then, we must drop the pretreating temperatureuntil the absorption factor just falls to 3X10 at which point thepretreaterefiluent will be a satisfactory feed as far as catalyst agingis concerned in the hydrocracking stages. A similar procedure will befollowed when the mixed crystallineamorphous base catalyst of FIG. 3 isused in the hydrocracking stage; only the pertinent numerical value ofthe tolerable light absorption factor will be different.

The quantitative nature, but not the qualitative nature, of thecorrelations of FIGS. 2 and 3 vary with the process conditions such aspressure, space velocities, hydrogen circulation rate and conversionlevels which were held fixed in generating the data of FIGS. 2 and 3.The quantitative aspects of the correlations also vary from catalyst tocatalyst, and for the correlation of FIG. 3, this is shown directly inFIG. 3. For the correlation of FIG. 2, this is shown in Table I in whichthe performance of two different commercial pretreating-stage catalystsare compared at substantially the same very low degree of aging, at 756F. and at the fixed process conditions of FIG. 2. The data for catalystA in Table I are taken from FIG. 2.

While temperature has been used as the controlled process variable inthe foregoing illustration of our invention, space velocity, hydrogencirculation and amount of condensed-ring polycyclic aromatics in thefeed passed to the pretreating-hydrogenation stage may suitably be usedas control variables in analogous ways.

In the processing combination herein described, the feed to thepretreating stage usually has a very much higher organically combinedoxygen, nitrogen and sulfur content than that of the subsequenthydrocracking stages. These nitrogen compounds (and ammonia made fromthem in the pretreater) absorb on the catalyst (probably at acid sites)and deactivate it. Some deactivation of hydrogenation-dehydrogenationsites of the catalyst also occurs by absorption of sulfur and/ornitrogen compounds on them. Therefore, the pretreating stage is run toachieve a relatively low level of hydrocracking, consisting primarily ofhydrogenolysis by ring opening of heterocyclic nitrogen, oxygen andsulfur compounds along with limited hydrogenolysis by hydrocracking ofhydrocarbons. The temperature necessary to get that kind of desiredconversion with the deactivated catalyst of the pretreating stage ismuch higher than it would be with the relatively undeactivated or higheractivity catalyst of the hydrocracking stage. At the relatively hightemperature of the pretreating stage, the hydrogenation activity of thedeactivated pretreating catalyst is very great. Thus, bydrogenation ofaromatics and unsaturates is the primary hydrocarbon reaction of thepretreating stage, and the data herein presented indicate that it maywell be the primary beneficial reaction of that pretreating stage.

In the hydrocracking stage, on the other hand, the concentration ofnitrogen and sulfur poisons which deactivate the catalyst isdeliberately held low, e.g. at 0.1 to 50 p.p.m. nitrogen in the feed ascompared with 100 to 2000 p.p.m. in the pretreating stage. If,therefore, the same'catalyst is used in this stage and in thepretreating stage, even the relatively high hydrocarbon conversiondesired in the hydrocracking stages will generally be achieved at'alower temperature than that employed for the relatively low conversiondesired of the poisoned catalyst in the pretreating stage. In essence,the pretreating stage is operated with a deactivated catalyst atsomewhat higher temperatures relative to the temperature of a subsequenthydrocracking stage wherein a catalyst of higher activity is employed.

There are circumstances under which it may be advantageous to operate asingle-stage process in which pretreating-hydrogenation andhydrocracking are conducted together at substantially the same processconditions. ,In the application of this invention to such a process, theamount of polycyclic aromatics in the feed stream, in the recycle liquidstream, or both are monitored as hereinbefore described, and used tocontrol one or more process variables such as temperature, spacevelocity, hydrogen circulation, amount of polycyclic aromatics in thefeed, end point of feed and conversion per pass so that thehydrocracking catalyst will not exceed a desired catalyst aging rate asexpressed in degrees F. per day or as catalyst life of at least about10,000 hours. If the polycyclic aromatics in these streams are monitoredby visible and ultraviolet absorption spectroscopy, then the pertinenttype of polycyclic aromatics being monitored contain 3 to 7 condensedrings.

Having thus provided a general description of the invention andpresented specific examples in support thereof, it is to beunders-tood'that no undue restrictions are to be imposed by reason ofthe specific examples presented except as defined by the claims.

TABLE L-EFFEC'I OF CATALYST N AROMATICSAND NITROGEN CONTENT OFPRETREATING-STAGE EFFLUENT i i [1000 p.p.m. N by weight in feed stock]Operating conditions:

Space Velocity, LHSV 1. 2 Pressure, p.s.i.g 2, 000

Designation of pretreating-stage catalyst A B Pretreating-stageefliuent:

Relative concentration of polycyclic aromatics containing 6 to 7condensed rings as indexed by ultraviolet absorption at 4,480 A.,absorption factor X 10. 9 9. 0 Organically combined N, p.p.m. by weight0. 25 0. 8

We claim:

1. In a process for converting a hydrocarbon feed fraction boiling inthe range of 400 to 1100. F. which includes the impurities comprisingsulfur, nitrogen and 16 polycyclic condensed ring compounds of three ormore rings, said process including the combination of a hydrogenationpretreatment step followed by a hydrocracking step, the improvement inoperating on-stream balance between the reaction conditions in saidpretreatment step and the concentration of said polycyclic condensedring compounds in said hydrocracking step, which comprises subjectingsaid hydrocarbon feed to hydrogenation pre treatmentconditions toconvert said nitrogen and sulfur impurities to ammonia and hydrogensulfide and to hydrogenate said polycyclics, controlling the severity ofsaid hydrogenating pretreatment conditions in response to a measureddetermination of the three to seven condensed ring polycyclic compoundsexisting in the hydrotreated effluent thereof so that a build-up of saidpolycyclic compounds will be avoided in the subsequent hydrocrackingstep and restricting the concentration of polycyclic compounds in saidefliuent to the hydrocracking step, said polycyclic compounds havinglight absorption characteristics in the range of 4000 to 5000 angstromsnot to exceed 350 p.p.m. and those having light absorptioncharacteristics in the range of 3000 to 4000 angstroms not to exceedabout 1000 p.p.m.

,2. The method of claim 1 wherein the catalyst in said hydrogenationpretreatment step comprises one or more hydrogenation componentsdispersed on an amorphous base having cracking activity in the range of20 to 45 and pores of a size in the range of 30 to 500 angstroms. 3. Themethod of claim 1 wherein the hydrogenation pretreatment step ismaintained at a temperature in the range of 650 to 800 F. employing ahydrogen partial pressure selected from within the range of 1500 to 2500p.s.1.g.

4. The method of claim 1 wherein the catalyst in said hydrocracking stepcomprises a crystalline aluminosilicate having a pore size in the rangeof 6 to 15 angstroms in admixture with an amorphous base crackingcomponent and being promoted with one or more hydrogenating components.

5. The method of claim 1 wherein the hydrocracking step is effected at atemperature selected from within the range of 550 to 750 F. employing ahydrogen partial pressure selected from within the range of 1000 to 2500p.s.1.g.

References Cited UNITED STATES PATENTS 3,132,086 5/ 1964 Kelley et al20857 3,166,489 1/ 1965 Mason et al. 20857 3,121,677 2/ 1964 Coggeshallet al 208178 3,152,980 10/ 1964 Coonradt et al. 20878 3,153,756 10/1964Williams et al. 324-.5 3,185,640 5/1965 Beavon 208134 3,219,574 11/1965Schneider 20857 3,269,939 8/ 1966 Marechal et al. 208143 3,384,5735/1968 Gorring 208-113 3,436,338 4/ 1969 Pratt et a1 208 DELBERT E. GANTS, Primary Examiner G. I. CRASANAKIS, Assistant Examiner US. Cl. X.R.20857

